The quality of your drinking water is important for your health, making bacterial testing a routine necessity, especially for the approximately 43 million people in the United States who rely on private well water. Unlike public water systems, private wells are unregulated, placing the full responsibility for water safety on the homeowner. Since harmful pathogens are often difficult and costly to test for individually, water quality is typically assessed by checking for indicator bacteria. The presence of these organisms, such as coliforms, signals a contamination pathway exists and that your water supply may be vulnerable to dangerous microorganisms. Regular testing is the only way to confirm your water is safe from bacteria that can cause illness.
Proper Sample Collection
Obtaining an accurate water sample is a precise process, and improper collection is the leading cause of false test results. You must use a sterile sampling bottle provided by the laboratory, which often contains a preservative like sodium thiosulfate to neutralize any chlorine present. This chemical ensures that bacteria are not killed in transit, preventing a false negative result. Never rinse the bottle, as this will wash out the preservative.
The most representative sample should be taken from an indoor, non-swivel, cold-water faucet that does not leak. Before collecting, remove attachments like aerators, screens, or hoses that can harbor bacteria and contaminate the sample. Disinfecting the faucet outlet with an alcohol swab or diluted bleach solution and allowing it to air dry is a recommended step.
Open the cold water tap fully and let the water run for two to five minutes to flush the line of stagnant water. Reduce the flow to a steady, pencil-sized stream that avoids splashing or touching the edges of the faucet. Carefully remove the bottle cap, holding it face-down, and fill the container to the designated fill line. The sample must be immediately refrigerated and delivered to the laboratory, ideally within 24 to 30 hours of collection, to maintain integrity.
Methods for Bacterial Water Analysis
The most reliable bacterial water analysis is performed in a certified laboratory using established scientific methods. The two primary professional techniques are the Membrane Filtration (MF) method and the Presence/Absence (P/A) test. The MF technique involves filtering a measured volume of water, typically 100 milliliters, through a fine membrane that traps the bacteria. The membrane is then incubated on a selective growth medium, allowing technicians to count the colonies and determine the concentration of bacteria present.
The P/A test is a simpler method that indicates whether the target bacteria are present or absent in the sample, often shown by a color change after incubation. While MF provides an exact count, P/A testing is often sufficient for monitoring drinking water quality, as the safety standard is typically zero bacteria. Both methods are accurate and are the standard for testing for Total Coliforms and E. coli.
At-home or do-it-yourself (DIY) kits are screening tools that offer a fast and inexpensive preliminary assessment. These kits rely on simple color-change strips or vials and lack the precision of laboratory equipment. Their main limitation is a lack of sensitivity, meaning they may only detect high concentrations of contaminants or produce false negative results. Due to the risk of user error and inaccurate readings, a positive result from a DIY kit should always be confirmed by a certified laboratory test.
Interpreting Test Results and Next Steps
Interpreting your test results requires understanding the two main indicator organisms: Total Coliforms and Escherichia coli (E. coli). Total Coliforms are a group of bacteria naturally found in the environment, including soil and decaying vegetation. Their presence in your water suggests a possible route for surface water contamination into your supply. A positive result for Total Coliforms alerts you to a potential problem with your well’s construction or integrity.
The presence of E. coli is more serious, as this organism lives exclusively in the intestines of warm-blooded animals and humans. Finding E. coli confirms recent fecal contamination, meaning that harmful pathogens such as viruses and parasites are also likely present, posing an immediate health risk. The safety threshold for both Total Coliforms and E. coli in drinking water is consistently “none detectable” or zero organisms per 100 milliliters of water.
If your test results are positive for either Total Coliforms or E. coli, you must immediately stop consuming the water. Temporarily, you can make the water safe for drinking, cooking, and brushing teeth by bringing it to a rolling boil for at least one full minute.
Shock Chlorination
The next step for well owners is shock chlorination, a one-time intensive disinfection process designed to kill bacteria throughout the well and plumbing system. This is done by introducing a high concentration of unscented household bleach into the well and circulating it through the plumbing until a chlorine smell is detected at all faucets. The highly chlorinated water must remain in the system for at least 8 to 12 hours to ensure proper disinfection. After this contact time, the system must be flushed completely until the chlorine odor is gone, and a retest should be performed a few days later to confirm the treatment was successful.